Successful agroforestry requires an understanding of the complex relationship between trees, crops and soils. This book provides a review of both social and biophysical aspects of soil use and research in agroforestry in the tropics, with an emphasis on nutrient-poor forest and savanna soils. Key topics covered include decomposition, nutrient cycling and leaching, soil structure, and soil biological processes. The book combines theory and a review of methods used in research.

Trees, Crops and Soil Fertility: Concepts and Research Methods

Forest restoration uses active management to re-establish natural forest habitat after disturbance. However, competition from early successional species, often aggressively invasive exotic plant species, can inhibit tree establishment and forest regeneration. Ideally, restoration ecologists can plant native tree species that not only establish and grow rapidly, but also suppress exotic competitors. Allelopathy may be a key mechanism by which some native trees could reduce the abundance and impact of exotic species. Allelopathy is a recognized tool for weed management in agriculture and agroforestry, but few studies have considered how allelopathic interactions may aid restoration. Here we introduce the “Homeland Security” hypothesis, which posits that some naive exotic species may be particularly sensitive to allelochemicals produced by native species, providing a tool to reduce the growth and impacts of invasive exotic species on reforestation. This article explores how exploiting allelopathy in native species could improve restoration success and the re-establishment of natural successional dynamics. We review the evidence for allelopathy in agroforestry systems, and consider its relevance for reforestation. We then illustrate the potential for this approach with a case study of tropical forest restoration in Panama. C4 grasses heavily invade deforested areas in the Panama Canal watershed, especially Saccharum spontaneum L. We measured the effect of leaf litter from 17 potential restoration tree species on the growth of invasive C4 grasses. We found that leaf litter from legume trees had a greater inhibitory effect on performance of S. spontaneum than did litter from non-legume trees. However, allelopathic effects varied greatly among species within tree functional groups. Further evaluation of intra- and inter-specific interactions will help to improve our selection of restoration species.

Allelopathy: a tool for weed management in forest restoration

This article summarizes allelochemicals' active variety, its potential causes and function mechanisms. Allelochemicals' activity varies with temperature, photoperiod, water and soils during natural processes, with its initial concentration, compound structure and mixed degree during functional processes,with plant accessions, tissues and maturity within-species, and with research techniques and operation processes. The prospective developmental aspects of allelopathy studies in the future are discussed.Future research should focus on: (1) to identify and purify allelochemicals more effectively, especially for agriculture, (2) the functions of allelopathy at the molecular structure level, (3) using allelopathy to explain plant species interactions, (4) allelopathy as a driving force of succession, and (5) the significance of allelopathy in the evolutionary processes.

Mechanism and Active Variety of Allelochemicals

The use of agroforestry systems in which pruning from trees is used to mulch the companion crops is an important area of research in the tropics. However, previous studies mostly evaluated the contribution of mulch to soil improvement and rarely examined the effect of mulch on weeds. Field experiments were conducted during the 1995 and 1996 growing seasons to investigate the effects of mulch from three woody fallow species on weed composition, biomass and maize grain yield. Treatments consisted of mulch from Leucaena leucocephala, Gliricidia sepium, and Senna siamea applied at rates of five and three tons dry matter ha−1 at planting and three weeks after planting (WAP), respectively, an unmulched treatment that received 90 kg N ha−1 of inorganic fertiliser, and an unmulched control plot that received no fertiliser. In both years and sampling dates, plots mulched with G. sepium and S. siamea had significantly lower weed density and biomass than the control plot in each of the sampling times and year of study. There was no significant difference in either weed density or biomass between the plot mulched with L. leucocephala and the unmulched plots. Mulches from G. sepium and S. siamea reduced weed density and weed biomass, while L. leucocephala was less effective in reducing weed biomass and weed density. Weed reduction by the mulches was in the order G. sepium ≥ S. siamea > L. leucocephala. Sedges were the dominant species in all the treatments except in G. sepium plots, where Talinum triangulare and other broadleaved species were dominant.

Selective control of weeds in an arable crop by mulches from some multipurpose trees in Southwestern Nigeria

The use of organic wastes as agricultural amendment is an effective strategy to regenerate degraded soils and represents an economical alternative that provides a local source of fertiliser. Olive mill wastewaters are widely known for their fertilising value. However, they can still not be recommended as a safe practice because of their potential ecological risks. Here, the effect of olive mill wastewaters on maize is investigated. Olive mill wastewater samples were collected in semi-modern and traditional mills of the Marrakech area, Morocco. Samples were analysed for pH, electrical conductivity and total phenolic content. Major phenolic compounds were identified and quantified. Phytotoxicity was assayed in the laboratory using maize seedlings, and under field conditions. Field fertilisation was conducted through land application accompanied or not by foliar spray according to a fractionated application mode. Our results show a high variability in physicochemical characteristics of wastewaters, notably for phenolic profiles. Variable toxicity levels were also attributed to each sample according to its salinity and phenolic load. Under field conditions, a 9% reduction of seedling emergence was observed in olive mill wastewater-pre-treated soil as compared with the control. However, olive mill wastewater-fertilised plots showed a net improvement in plant height of 10–11% compared with the control. A yield improvement reached 28% over that of the control when the plants were amended both by land application and foliar spray. These results show that the large variability in olive mill wastewaters composition constitutes a major limiting factor for their re-use in agricultural systems. Olive mill wastewaters can be used as a safe agronomic amendment provided the application is moderate and progressive and critical development stages are avoided, notably the preliminary development stages. Foliar fertilisation using olive mill wastewaters is also revealed to be a promising way to manage these effluents.

/pdf/phytotoxicity-and-fertilising-potential-of-olive-mill-4wu07lxusb.pdf

Phytotoxicity and fertilising potential of olive mill wastewaters for maize cultivation

Leaf extracts and mulch from 14 multipurpose trees were used to test their effects on maize germination, growth and yield. Maize germination was significantly reduced by leaf extracts of all species with increasing extract concentration. The most drastic reductions were caused by Gliricidia sepium, Tetrapleura tetraptera, Lonchocarpus sireceus, Senna siamea and Leucaena leucocephala. Terminalia superba, Tetrapleura tetraptera, Pithecelobium dulce, Gliricidia sepium and Senna siamea significantly reduced maize root growth at the lowest extract concentration, while shoot length was most significantly reduced by Gliricidia sepium, Leucaena leucocephala, Alchornea coordifolia, Pithecelobium dulce, Terminalia superba, and Tetrapleura tetraptera at all concentrations. Growth of maize in pots and yield in the field were controlled by nitrogen supply and uptake. Fast-decomposing leaf material rich in nitrogen promoted growth and yield of maize. Thus Leucaena leucocephala and Gliricidia sepium, both nitrogen-fixing legumes with high nitrogen contents and fast-decomposing leaf materials, significantly increased maize growth and yield. This suggests that the negative effects of leaf extracts observed in the laboratory are ecologically irrelevant in the field.

Zusammenfassung

Blattextrakte und Mulch von vierzehn Mehrzweckbaumen wurden verwendet, um deren Einflusse auf das Maiskeimverhalten, das Wachstum und den Ertrag von Mais zu untersuchen. Die Keimung von Mais wurde durch die Blattextrakte aller Arten mit zunehmender Extraktkonzentration reduziert. Die starkste Reduktion wurde durch Gliricidia sepium, Tetrapleura tetraptera, Lonchocarpus sireceus, Senna siamea und Leucaena leucocephala verursacht. Terminalia superba, Tetrapleura tetraptera, Pithecelobium dulce, Gliricidia sepium und Senna siamea reduzierten signifikant auch bei geringster Extraktkonzentration das Maiswurzelwachstum, wahrend die Sproilange bei allen Konzentrationen am starksten durch Gliricidia sepium, Leucaena leucocephala, Alchornea coordifolia, Pithecelobium dulce, Terminalia superba und Tetrapleura tetraptera bei allen reduziert wurde. Das Wachstum von Mais in Gefaien und der Ertrag im Feld wurde durch die Stickstoffzufuhr und -aufnahme kontrolliert. Der schnelle Abbau von Blattmaterial mit hohem Gehalt an Stickstoff forderte Wachstum und Ertrag von Mais. Daher fuhrte Leucaena leucocephala und Gliricidia sepium, Stickstoff-fixierende Leguminosen mit hoheren Stickstoffgehalten und rasch abbauendem Material, signifikant zu einer Verstarkung des Maiswachstums und Erhohung des Ertrages. Diese Reaktion macht die negativen Wirkungen der Blattextrakte, die im Laboratorium beobachtet wurden, okologisch bedeutungslos im Feldanbau.

Effect of Mulch from Selected Multipurpose Trees (MPTs) on Growth, Nitrogen Nutrition and Yield of Maize (Zea mays L.)

Leaf extracts and mulch from 14 MPTs were used to study their effects on germination, initial growth and modulation of cowpea. Aqueous leaf extracts of 1/4, 1/2, 3/4 and full concentrations were added to cowpea seeds placed in Petri dishes. In the pot experiment, cowpea was planted in plastic pots filled with 1200 g of soil, and mulch was either surface-applied or incorporated into the substrate at rates equivalent to 5. 10 and 20 tons.ha -1 . Leaf extracts from Gliricidia sepium, Senna siamea, Milletia thoningii, Grewia pubescens and Tetrapleura tetraptera reduced cowpea germination significantly at concentrations above 25 % of the full concentration 7 days of incubation. Root length was also strongly reduced by extracts from Gmelina arborea, Senna siamea hornea coordifolia, and Tetrapleura tetraptera. These inhibiting effects were also observed in the pots. Mulch incorporation significantly reduced growth and modulation of cowpea in the pots. These reductions were more pronounced with the incorporation of mulch from fast decomposing trees such as Leucaena leococephala, Gliricidia sepium and Grewia pubescens Although Leucaena leucocephala and Gliricidia sepium relcased much nitrogen, they strongly reduced cowpea growth when incorporated. This may suggest that other reasons than N immobilization caused growth reduction when the plant residues were applied to cowpea.

Effects of mulch from 14 multipurpose tree species (MPTs) on early growth and nodulation of cowpea (Vigna unguiculata L.).

Planted fallows of seven multipurpose tree species were analysed in order to reveal their impact on understorey vegetation in South-Western Nigeria. Although having light environment under their canopy comparable with most of the other tree species, Nuclea latifolia and Pterocarpus santalinoides had the lowest density and biomass of understorey vegetation. Only Milletia thoningi had lower understorey biomass than these two tree species although it had higher understorey density. Soil fertility was not a limiting factor for the undergrowth since those three species which had the highest undergrowth density and biomass had either comparable or lower fertility status. This implies that, the suppressive effect of Nuclea latifolia and Pterocarpus santalinoides the understorey growth might have been due to allelopathic effects caused by secondary metabolites leached from the tree canopy during precipitation or from the floor litter during decomposition. Because of the limitations imposed by the lack of randomisation of the tree stands in the site, conventional analysis of variance was not appropriate. Instead, the calculated means for each variable was used to compare the results.

https://www.jarts.info/index.php/tropenlandwirt/article/download/1492/668

N. Sanginga

Papers

Effect of Mulch from Selected Multipurpose Trees (MPTs) on Growth, Nitrogen Nutrition and Yield of Maize (Zea mays L.)

Effects of mulch from 14 multipurpose tree species (MPTs) on early growth and nodulation of cowpea (Vigna unguiculata L.).

Comparisons of Understorey Vegetation in Planted Fallows of Seven Multipurpose Tree Species (MPTS) in South-Western Nigeria